An anniversary gift carries a date, which is the whole opportunity. A piece she wears daily becomes a quiet reminder of that date every time she puts it on.
Choose for the wardrobe she has now, not the one she had when you met. People's taste in metal and scale drifts; buy for who she is this year.
Should I engrave the date?
If you engrave anything, the date is the right thing — it is specific and it ages well. Keep it small and on the inside or back of the piece so it stays private.
Is it better to go bigger each year?
No. Escalating every year turns a gift into a contest with your own past. Choose the piece that fits this year's life — what she wears, where she goes — and let the note carry the weight.